In view of the world consumption increase of fresh produce and the concern with their quality due to their low resistance to handling and transportation, post harvest perishability and susceptibility to microbiological contamination, investigations on new technologies to reduce losses and maintain its organoleptic characteristics are of fundamental importance. The objective of this work was to evaluate the effect of ozone gaseous mixture in reducing microbiological load of iceberg and green-leaf lettuces. The leaves were exposed to an ozone mixture flow in three different concentrations: 1.74; 4.98 and 8.34 ppm for 10, 30 and 50 seconds, so that at least two treatments reached the final concentration of 0.06, 0.10 and 0.30 ppm within the chamber. After application of the treatments, the American and curly leaves lettuces were kept in BOD and selected for analysis at 24, 72 and 120 hours after exposure to ozone. Microbiological analyzes were performed according to the official American Public Health Association methodology, with a serial decimal dilution method for count of colony forming units (CFU) of mesophilic aerobes, psychrotrophs, molds and yeasts, and the most probable number method (MPN) for the presence of total and thermotolerant coliforms at 45ºC. No presence of Salmonella spp. and Escherichia coli on the iceberg and green-leaf lettuce were detected. With the application of the gaseous ozone mixture there was a significant reduction in the levels of aerobic mesophilic and psychrotrophic bacteria, molds and yeasts present on the surface of the leaves. Even at concentrations considered low, 0.06 and 0.10 ppm, the ozone mixture was effective in reducing most of the considered microorganisms and in maintaining the quality characteristics of the lettuce leaves.